Calculating the risk of infection of mycobacterium tuberculosis in endemic settings

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Calculating the risk of infection of mycobacterium tuberculosis in endemic settings

Johnstone-Robertson, Simon Peter

2012-03

ENGLISH ABSTRACT: The annual risk of infection (ARI), a measure of recent transmission, has been described as the most
important parameter in tuberculosis (TB) epidemics. Nevertheless, mounting evidence suggests all
factors contributing to TB transmission are not yet completely understood. This research was
performed to investigate the role various parameters, e.g. overcrowding, period of infectivity,
ventilation, and infectivity of source cases, play in TB transmission. An established airborne
transmission risk model, the Wells-Riley equation (WRE), was modified to account for scenarios
where unknown numbers of infectious individuals may be present. Subsequently, the ARI for three
indoor locations conducive to TB transmission were calculated. Two locations (households and
minibus taxis) were identified in a social mixing survey conducted within a South African
community where TB is endemic as a part of this research. The third location (prison) was
identified in an earlier independent study in the same community. The impact various interventions
could have in reducing the ARI associated with each location was explored. Poor ventilation, severe
overcrowding, extended exposure periods, and high incidence rates contributed to high TB
transmission risks in each location. The household-associated ARI was related to the number of
resident adults. Current TB control programs will only reduce the ARI if household ventilation
levels are improved simultaneously. Similar reductions in the ARI could be achieved by trebling
current ventilation levels or by separating child and adult sleeping areas. Neighbouring households
can also contribute substantially to the ARI. The minibus taxi-associated ARI for drivers and
commuters was considerable but readily reduced by opening windows or keeping the fresh-air fan
on. Reducing TB case prevalence through active or passive case-finding would reduce the ARI
substantially. The prison-associated ARI was proportional to levels of overcrowding. No single
intervention, such as improved ventilation, decreased lock-up time, or improved case-finding,
would decrease the ARI substantially, but concurrent implementation of all of them to meet national
or international standards would. This research shows TB is not only transmitted in epidemics by
highly infectious TB cases, but that any TB case, no matter how infectious, has the potential to
infect susceptible people under the right conditions.